Today in history… bridge collapse sparks ‘break step’ rule

12:00am Apr 12, 2018 | Admin - Acorn Stairlifts UK & Lifestyle

Visit many of Britain’s historic bridges and you will find at either end a notice instructing troops to “break step” when marching across.

The catalyst for this warning was an incident which happened on April 12th, 1831, at the Broughton Suspension Bridge over the River Irwell. Built in 1826, it connected the communities of Broughton and Pendleton, now in Greater Manchester, and was one of the first suspension bridges in Europe.

The 114ft bridge was paid for by local industrialist John Fitzgerald and was the only connection between the two towns on opposite banks of the Irwell. The new Menai Suspension Bridge, connecting mainland Wales with Anglesey and designed by renowned engineer Thomas Telford, had opened that same year. Suspension bridges were considered ‘a wonder of the modern age’ and the smaller but still impressive bridge over the Irwell was a source of great local pride.

It was an iron chain suspension bridge attributed to an early pioneer of their design, Samuel Brown, although other sources suggest it was built by Manchester millwright and textile machinery manufacturer Thomas Cheek Hawes. On the day of its grand opening local people flocked onto the new bridge and over the next few years it carried a steady passage of pedestrian and vehicular traffic, all without any sign of a problem.

But disaster struck 187 years ago today in an incident which would affect suspension bridges around the world. A detachment of 74 troops from the 60th Rifle Corps had been out on exercise and were marching in column four abreast back to their barracks in Salford.

As the column crossed the bridge the soldiers noticed it vibrating in time with their marching footsteps, moving up and down like a sprung dance floor. Amused by the unusual sensation, some began whistling a tune and marching in a more pronounced manner to increase the vibration. With the whole of the column on the bridge and its head having almost reached the Pendleton side, the men suddenly heard a number of loud cracking sound, which some likened to sporadic gunshots.

Suddenly, one of the iron columns supporting the suspension chains at the Broughton side of the river fell inwards towards the bridge, still bolted to a large stone plinth from the pier supporting it. No longer supported by its suspension chain, that corner of the bridge then collapsed about 18 feet into the river, throwing about 40 of the soldiers into the water or against the wreckage.

Mercifully, the river was low and only about two feet deep at that point. None of the men were killed, but about 20 were injured, several suffering broken arms and legs or cuts and bruises. The rest broke ranks and rushed off the bridge, fearing a total collapse, before going to the aid of their stricken comrades.

An investigation into the collapse found that a bolt had snapped on one of the stay chains securing the iron column to the stone pier, while other bolts were bent but not broken. The failed bolt was found to be badly forged and would have eventually broken anyway if not replaced, but crucially the investigation found it was the soldiers’ rhythmic marching which hastened the bolt’s failure.

It had set up a ‘harmonic oscillation’ in the bridge, essentially causing it to vibrate in time with their footsteps and repeatedly stressing then relaxing its support structure. Following the investigation, an order was issued throughout the British armed forces that whenever troops were crossing a bridge, especially one prone to vibrate or sway, they should ‘break step’, in other words, not march in time but just walk normally.

The accident also caused a minor loss of confidence in suspension bridges, with one newspaper suggesting even the mighty Menai Bridge might collapse “if a thousand men were to be marched across in close column and keeping regular step”. Thankfully it has stood the test of time.

Other nations’ armed forces also adopted the ‘break step’ rule for crossing bridges, but troops, after many hours being drilled to march in step, found it very difficult not to when walking in column. Despite the rule, marching was cited as a contributing factor in the collapse of the Angers Bridge, in France, which killed more than 200 soldiers.

The Broughton Suspension Bridge was rebuilt and strengthened (as pictured above) and served another 93 years before being replaced with a footbridge in 1924, which is still in place. By then, vehicles used other more modern bridges.